The invention relates to new traceless linkers which use transition metal complexes to link a xcfx80-orbital containing substrate for subsequent synthesis by, for example, combinatorial chemistry or multiple parallel synthesis (MPS) to a support.
Combinatorial chemistry and MPS are techniques that are of great value in the efficient production of large numbers of molecules. Such large collections of molecules are of use in screening for useful properties or effects. The development of combinatorial chemistry and of MPS has been greatly facilitated by the use of solid-phase-synthesis in which substrate molecules are covalently attached to a solid support. The product of the reaction can be isolated by mechanically separating the solid support from the other components of the reactions and the product separated from the solid support by, for example, chemical cleavage. The whole process can be carried out quickly and efficiently and in many cases can be automated.
Members of a set of molecules made by combinatorial chemistry or MPS will have a recognisable common framework inherited from each different substrate and optionally modified during the combinatorial chemical steps. Disposed about the framework are the variable parts that derive from combinatorial modification of the substrate, for instance by combinatorial refunctionalisation of its functional groups. It is the variation in these parts that determines the diversity of the collection of molecules.
In most examples of solid-phase-synthesis the substrate is attached to the solid support through a covalent bond formed by functional groups on the substrate and support, for example by the formation of a carboxamide group between the support and the substrate.
This has three main limitations:
1. The functional group that is used to bind the substrate could have been used to introduce further variable reagents. This lost opportunity is a combinatorial reduction in the potential diversity of the molecules produced.
2. Although the object is to produce sets of compounds of great diversity in order to screen for useful properties or effects, the functional group that is linked to the support is a feature that all members of the combinatorial set of compounds share so that the diversity of the compounds produced is compromised by this common feature.
3. The chemistry required to attach a molecule through a functional group to a support varies as the functional group varies. In general, each different functional group will need to be linked to a support that has been specially prepared to react with it. Each differently linked functional group will need a different chemical treatment to release it from the support. Each different linking functional group will place different restrictions on the type of chemistry which can be carried out on the supported molecules.
A number of attempts have been made to link substrates to supports by linkers which are designed to be traceless, i.e. the linker leaves no feature on the final product. Traceless linkers are those that are not being used as a protecting group for a specific functional group, and that are removed from the product molecules as part of the process of releasing the molecules from the support. The traceless linkers reported so far (e.g Sucho Leiki I. Tetrahedron Lett., (1994), 35, 7307: Chenera B et. al., J. Am. Chem. Soc., (1995), 117, 11999: Plunkett, M. S. et. al., (1995), 60, 6006) have been developed for specific applications and are either of limited generality or require specific chemical synthesis of every variation of the first supported reagent, or leave behind a characteristic functional group that limits the diversity of the molecules produced. For instance, the existing examples of acid labile arylsilane linkers or reductively cleavable alkylthioether linkers are limited in scope and synthetically limited in accessibility. Acid labile arylsilane linkers are obtained by separate functionalisation or refunctionalisation of an aromatic ring of each linked substrate and after acid-induced cleavage of the linker every product contains a hydrogen atom at the position previously occupied by the silane functional group. The alkylthioether linkers similarly require a separate functionalisation or refunctionalisation of each linked substrate and cleavage of the thioether functional group that constitutes the link results in a reduction of the degree of functionality of the product.
Organometallic chemistry is a subset of chemistry dealing with the area of metals containing ligands bonded through a carbon atom. The area is extremely diverse due to the number of different interactions possible with the metal group, thus allowing any number of diverse compounds to be produced. One form of interaction between a metal and an organic group is in the form of a xcfx80-complex between an organic ligand having a xcfx80-bond, such as is present on an olefinic or an aromatic compound, and a transition metal. A characteristic of such complexes is that the bond between the transition metal and the organic group is formed to the xcfx80-orbitals of the organic group and not the "sgr"-orbitals of the framework. Although the reactivity of the xcfx80-orbitals is thereby changed the connectivity of the framework of the organic group remains intact. Such xcfx80-bonded transition metal complexes constitute a well-known class of compounds that is described, for instance, in xe2x80x9cComprehensive Organometallic Chemistryxe2x80x9d Wilkinson G., Stone G. A., and Abel E. W. Eds., Pergamon Press, 1982, Oxford, U.K. and in xe2x80x9cTransition Metals in the Synthesis of Complex Organic Moleculesxe2x80x9d Hegedus L. S., University Science Books, 1994, Mill Valley, Calif., USA. Examples of such complexes with common organic compounds include (1) (xe2x80x9cComprehensive Organometallic Chemistryxe2x80x9d Vol. 3, Chapter 26.2, Tables 10 and 11 1003-1005) and (2) (xe2x80x9cTransition Metals in the Synthesis of Complex Organic Moleculesxe2x80x9d Chapter 7.3, 221). 
Transition metal complexes serve as masking or protective groups for xcfx80-bonded ligands of organic compounds. Soluble xcfx80-bonded complexes of arenes with chromium(0), of alkynes with cobalt(0), dienes and enones with iron(0) and olefines with iron (+1) are well known and have been used as protective groups for organic groups during functional group manipulation.
In addition xcfx80-bonded transition metal complexes have been immobilised on solid polymer supports via a polymer attached phosphine ligand for use as catalysts in organic and polymer chemistry. The polymer support is needed so as to aid isolation of the catalyst from the reaction mixture.
We have now found that transition metal complexes can be used as traceless linkers of remarkable simplicity, generality, and utility. These transition metal complexes may be used to xcfx80-bond unsaturated organic substrate ligands, such as arenes, alkynes, alkenes, and dienes, to a support for use in combinatorial chemistry and MPS.
The invention is illustrated below. 
A feature of the traceless linker illustrated is its versatility: not only can the transition metal attach to a wide variety of xcfx80-orbital containing substrates, to allow an enormous diversity of library components to be synthesised, but it can also be attached to a great variety of supports. The transition metal+linker ligand constitutes a xe2x80x9ctracelessxe2x80x9d linker because no trace of the link in the form of a characteristic functional group remains on the chemically modified substrate molecule after it has been released, as the product, from the support. This is true also in cases where the original xcfx80-orbital containing framework (an unsaturated system) of the substrate ligand has itself been modified during the synthesis or cleavage of the product since the modification to the original xcfx80-orbital containing framework is a modification to the core or backbone of the molecule rather than to its functional groups. Therefore all the functional groups of a substrate supported in this way are available for diverse modification.
Presented as a first feature of the invention is the use of a transition metal complex as a traceless linker between a support and an unsaturated organic molecule by the formation of a xcfx80-complex bond between the transition metal and the unsaturated organic molecule and attaching of the transitional metal to the support before during or after the formation of the xcfx80-complex bond, which unsaturated organic molecule is then, preferably, a substrate for further chemical modification and eventual release from the traceless linker.
A further feature of the invention is a traceless linker system which comprises a transition metal complex attached to a support which is capable of forming a xcfx80-complex bond with an unsaturated organic molecule.
A further feature of the invention is a traceless linker system which comprises a transition metal complex attached to a support and an unsaturated organic molecule attached to the transition metal through a xcfx80-complex bond.
A further feature of the invention is a method of attaching to a support via a transition metal complex traceless linker an unsaturated organic molecule, which is then, preferable, a substrate for further chemical modification and eventual release from the transition metal complex, which method comprises one of the following alternative first steps:
(1) bonding the transition metal complex, which has bound to it through a xcfx80-complex bond the unsaturated organic substrate molecule, with the support, where the transition metal complex, support or both has a group capable of forming a bond, linker or interaction between the transition metal complex and the support;
(2) bonding the unsaturated organic substrate molecule with the transition metal complex, which is bound to the support, by forming a xcfx80-complex between the supported transition metal and the unsaturated organic substrate; or
(3) converting an organic molecule which is "sgr"- or xcfx80-bonded to the transition metal, which is bound to the support, to form a xcfx80-complex between the supported transition metal and the unsaturated organic substrate molecule.
Preferably specific preferred later steps include subsequent synthesis on the bound unsaturated organic substrate, where synthesis may be either by combinatorial chemistry or by multiple parallel synthesis techniques, and removal of the product from the support.
Thus formed is a xcfx80-substrate-transition metal-support complex in which the xcfx80-ligand, or ligands, is the substrate for a chemical reaction, and at least one of the other ligands of the complex is the linker group between the transition metal xcfx80-complex and the support. Examples of suitable complexes, where L is the linking ligand and S is the support, are:
1. xcfx80-Arene Cr(CO)2L-S complexes as supported arene substrate.
2. xcex72-Alkyne Co2(CO)5L-S complexes as supported alkyne substrate.
3. xcex72-Alkene FeCp(CO)L+-S and xcex74-diene Fe(CO)2L-S complexes as supported olefin substrate.
Where Cp=xcex75-cyclopentadienyl anion
The support may be insoluble, such as a polymer or resin, soluble, such as a polyethylene glycol (PEG) which can be selectively precipitated as required, or fluorous phases, which show temperature dependent immiscibility with common organic solvents. It will be understood that it is possible to complex the unsaturated organic molecule to the transition metal at any stage, for example it may be convenient to build the transition metal complex onto the support prior to complexation with the unsaturated organic molecule or to complex the unsaturated organic molecule with the transition metal prior to attachment to the support. Alternatively the xcfx80-unsaturated organic molecule transition metal comple may be formed in situ by conversion of, for example, a "sgr"-organic molecule transition metal complex.
The substrate ligand may be any unsaturated organic molecule containing at least one functional group that can be modified in a chemical synthesis and a xcfx80-orbital system that is capable of forming a xcfx80-complex with a suitable transition metal complex. The xcfx80-orbital system is preferably an integral part of the structural framework of both the substrate and the product rather than of one of the functional groups of the substrate or product.
The linking group can be any functional group capable of complexing with the transition metal and joining to the support. The linker group is preferably selected from those ligands known to form a strong bond to the transition metal. Suitable ligands include phosphines, phosphites, and isonitriles. A further embodiment of the reaction is that a single support bearing a common linking ligand can be used to support a wide variety of substrates through different transition metal complexes.
An additional embodiment of the invention is that the transition metal xcfx80-complex may itself take part in the chemical transformation. One example occurs where the formation of the complex of the unsaturated fragment (a xcfx80-system) of the substrate changes the reactivity of neighbouring functional groups to facilitate reactions that would otherwise be difficult to achieve; for instance the ready displacement of an aryl halide by diverse nucleophiles in xcfx80-arene Cr(CO)2L complexes. A further example occurs where the unsaturated fragment (a xcfx80-system) of the substrate ligand itself undergoes a chemical reaction to produce a new unsaturated fragment (a xcfx80-system) of a modified substrate; for instance in the reaction of [xcex75-dienylFe(CO)2L]+ complexes with carbanions to give xcex74-dieneFe(CO)2L complexes. A further example occurs where the unsaturated fragment (a xcfx80-system) of the substrate ligand itself undergoes a chemical reaction to produce a modification of the unsaturated fragment that no longer complexes to the supported transition metal complex so that concomitant release of the product ligand occurs to give the unsupported product; for instance in the Paulson-Khand reaction of xcex72-alkyne Co2(CO)5L complexes to give cyclopentenones.
A suitable transition metal is one capable of forming a cleavable stable xcfx80-complex contemporaneously with the xcfx80-orbital containing substrate and other ligands, at least one of which (the linker ligand) is able to be covalently attached to the solid support. The preferred transition metal varies according to the nature of the substrate ligand. Factors affecting the choice of suitable transition metal are known to those skilled in the art and are described inter alia in xe2x80x9cComprehensive Organometallic Chemistryxe2x80x9d Wilkinson G., Stone G. A., and Abel E. W. Eds., Pergamon Press. 1982, Oxford, U.K.
Another additional embodiment of the invention is that when a transition metal complex linked to the support contains at least one exchangeable ligand other than the linker and substrate ligands, this exchangeable ligand can be replaced in part at each stage of the chemical modification of the substrate ligand by treatment with trace quantities of a tagging ligand. The unique combination of tagging ligands thus specifies the chemical history of the substrate linked to the support and acts as a label for e.g. resin beads in combinatorial split synthesis.
The tagging ligands are chosen from a set of similar but diverse compounds such as triaryl phosphines or halogenated alkyl or aryl phosphites, where each tagging molecule is unique to each of the different chemical modification steps used in the overall synthesis.
The tagging techniques described above may be practised not only on the transition metal complex linking the substrate to the support but to additional transition metal complexes on the support added exclusively for tagging. Alternatively the tagging techniques described above may be used alongside other linker techniques. Suitable tagging ligands are detectable and identifiable by mass spectrometry.
Presented as a further feature of the invention is a method for tagging a support within a chemical synthesis comprising for each reaction step desired to be labelled contacting a transition metal complex containing an exchangeable ligand bound to the support with a tagging ligand unique for the reaction being performed.
Presented as a further feature of the invention is a method for tagging a support within a chemical synthesis comprising for each reaction step desired to be labelled contacting a transition metal complex with a reactive group capable of making an analysable modification on one of the ligands of the transition metal complex unique for the reaction being performed.
It will be understood that the tagging reagents, whether a tagging ligand or reactive group, are introduced in such a way that only a small fraction of the sites that may be tagged are modified in any single step. Used in this way several different tagging reagents may be added separately and sequentially to unambiguously identify each of a sequence of several chemical reactions without any of the tagging reagents significantly interfering with the effectiveness of any of the other tagging reagents or with the chemistry being carried out on the supported xcfx80-complex of the substrate.
A cleavable stable xcfx80-complex is one in which the complex of the substrate ligand and the transition metal is stable to storage and use and from which the product ligand can be released. Preferably the unsaturated fragment (a xcfx80-system) will be unchanged. Factors affecting the choice of suitable cleavable complexes are known to those skilled in the art and are described inter alia, in xe2x80x9cComprehensive Organometallic Chemistryxe2x80x9d Wilkinson G., Stone G. A., and Abel E. W. Eds., Pergamon Press, 1982, Oxford, U.K.
The product ligand (unsaturated organic molecule with modified functional groups) may be cleaved from the complex to give the product by any effective method that does not damage the structural integrity of the product. Suitable methods include thermal, photochemical, and oxidative cleavage.
The substrate transition metal xcfx80-complex is located on the surface or throughout the support in such a way that it is accessible to the constituents of a reaction that is to be carried out on the substrate ligand.
A-xcfx80-bonded transition metal complex with an exchangeable ligand can be preformed and attached to a supported ligand to give a supported xcfx80-bonded transition metal complex, by a simple process of ligand exchange well known to those skilled in the art.
A transition metal complex with exchangeable ligands can be attached to a supported ligand to give a supported transition metal complex with exchangeable ligands and these ligands can be replaced with the substrate ligand (unsaturated organic molecule) with an unsaturated fragment (a xcfx80-system), another simple process of ligand exchange well known to those skilled in the art.
A-xcfx80-bonded transition metal complex with a functionalised linking ligand can be preformed and attached to a support by refunctionalisation of the functionalised linking ligand to give a supported xcfx80-bonded transition metal complex, for instance by coupling a carboxy functionalised linking ligand to an amino-functionalised support by amide bond formation.
A supported transition metal complex containing a ligand may be chemically modified so that the ligand is converted into a xcfx80-bonded substrate ligand of a xcfx80-bonded transition metal complex; for instance in the conversion by Doetz reaction of supported "sgr"-carbeneCr(CO)4L complexes into xcfx80-arene Cr(CO)2L complexes of supported arenes ("sgr"-complex to xcfx80-complex).
A supported transition metal complex containing a xcfx80-bonded ligand may be chemically modified so that the xcfx80-bonded ligand is converted into a xcfx80-bonded substrate ligand; for instance in the conversion of [xcex75-dienylFe(CO)2L]+ complexes into xcex74-dieneFe(CO)2L complexes (xcfx80-bonded complex to another xcfx80-bonded complex).